1. Field of the Invention
The present invention relates to an internal combustion engine with a dual turbocharger system having a first turbocharger and a second turbocharger in which air is charged to the engine by only the first turbocharger at small to medium intake air quantities and is charged by both the first and the second turbochargers at large intake air quantities.
2. Description of the Prior Art
As illustrated in FIG. 11 (Prior Art), a conventional turbocharged internal combustion engine 1' is provided with a first turbocharger 7' and a second turbocharger 8' having turbines 7a' and 8a', respectively, and compressors 7b' and 8b', respectively. An exhaust switching valve 17' is installed in an exhaust conduit 20b' downstream of the turbine 8a'. Similarly, an intake switching valve 18' is installed in an intake conduit 14b' downstream of the compressor 8b'. Further, an intake bypass valve 33' is installed in an intake bypass conduit 13' bypassing the compressor 8b'. The intake switching valve 18' and the exhaust switching valve 17' are closed at small to medium intake air quantities to produce "one-turbocharger-operation" wherein only the first turbocharger 7' is operated. The intake switching valve 18' and the exhaust switching valve 17' are opened at large intake air quantities to produce "two-turbocharger-operation" wherein both the first turbocharger 7' and the second turbocharger 8' are operated. A smooth transition from a "one-turbocharger-operation" to a "two-turbocharger-operation" is obtained by partially opening the exhaust switching valve 17' to run-up the second turbocharger 7'. In the conventional turbocharger system, the exhaust switching valve 17' begins to be partially opened at a substantially constant charging pressure and is opened at a substantially constant opening speed whether the transmission coupled to the engine is at a low gear position or at a high gear position.
However, partially opening the exhaust switching valve as described above can degrade system performance. More particularly, because the exhaust switching valve 17' is partially opened at a constant pressure, a large shock is likely to occur during transition from the "one-turbocharger-operation" to the "two-turbocharger-operation" when the transmission is in low gear. Also, because the exhaust switching valve 17' is opened at a constant opening speed, either an over-charging at low transmission gear positions or a charging pressure decrease at high transmission gear positions is likely to occur during transition from the "one-turbocharger-operation" to the "two-turbocharger-operation". Further, the transition characteristic is affected by not only the opening speed of the exhaust switching valve but also an engine speed changing rate which is in turn affected by a road condition.